Explanation

The special properties exhibited by substances near critical points. The physical phenomena near the critical point have a series of characteristics. From the point of view of thermodynamics, the coexistence of liquid and gas corresponds to the equal chemical potential of the liquid phase and the gas phase. The unequal specific volume (reciprocal of density) of the two phases means that the first derivative of the chemical potential of the two phases to the pressure is unequal during the phase change; The critical point is the point where the first derivative of the chemical potential of two phases to the pressure is also equal. In the thermodynamics theory of phase transformation, the phase transformation with discontinuous first derivative of the chemical potential during phase transformation is called first-order transformation, and the phase transformation with continuous first derivative is called second-order transformation, or continuous transformation. So critical phenomena refers to the phenomenon near the continuous phase transition point. There are many continuous transformation in solids, such as the iron paramagnetic phase transformation at the curie point, the anti iron paramagnetic phase transformation at the Nail point, the ferroelectric paraelectric phase transformation at the curie point, and the normal superconducting phase transformation without an external magnetic field. Most of the order-disorder transformation is a continuous, cooperative second-order transformation, without latent heat effect. There is also a small number of order disorder transformation to first-order transformation, which is discontinuous and has latent heat effect. Continuous phase transformation is often the change of symmetry of the system, such as the change of space group of lattice in displacement type structural transformation; Magnetic phase transition is the change of crystal magnetic group; The superconducting phase transformation is a change of gauge symmetry. Generally, one or several order parameters can be defined to describe the continuous transformation. It (or they) is zero in one phase (usually the phase with high symmetry), is not zero in the other phase (usually the phase with low symmetry), and is zero at the phase transformation point: the phase transformation timing parameters change continuously. In the phase transformation of gas-liquid critical point, the order parameter can be the difference of two-phase density or the difference of specific volume; The magnetization of solid can be selected in the ferromagnetic paramagnetic phase transformation.

Classification